Concrete Slab Calculator

What this calculator does

This concrete slab calculator focuses on flat, on-grade pours: patios, driveways, sidewalks, shed floors, and equipment pads. Enter length, width, and thickness in your preferred units, set a waste percentage, and the calculator returns volume in cubic yards, cubic feet, and cubic meters, plus 40 lb, 60 lb, and 80 lb bag counts. Add a price per cubic yard for a cost estimate. For footings, columns, walls, or stairs, use the main Concrete Calculator.

Formula

Volume: V_ft³ = length_ft × width_ft × thickness_ft

Multi-pad totals: V_total = V_one × quantity

Order with waste: V_order = V_total × (1 + waste)

Conversions: V_yd³ = V_ft³ ÷ 27 · V_m³ = V_ft³ × 0.0283168

Bag count: bags = ⌈V_ft³ ÷ bag_yield × (1 + waste)⌉ · 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³.

Cost: cost = V_order_yd³ × price_per_yd³

Variable definitions

• length / width: Slab footprint in your chosen unit, converted to feet internally.
• thickness: Slab depth, typically entered in inches (4 in for patios/sidewalks, 5–6 in for driveways and pads).
• quantity: Number of identical slabs to pour (e.g., 4 deck-post pads).
• waste: Extra ordered for over-pour and form variation, as a decimal (0.05 = 5%, 0.10 = 10%).
• bag yield: Cured volume per bag: 0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb, 0.60 ft³ for 80 lb (Quikrete / Sakrete equivalents).

Step-by-step calculation

1. Pick a preset (patio, sidewalk, driveway…) or enter custom dimensions.
2. Confirm units for length, width, and thickness, the calculator converts to feet internally.
3. Enter quantity if you're pouring multiple identical pads.
4. Set the waste factor, 5% for clean rectangles on a flat sub-base, 10% for irregular forms.
5. Optionally enter price per cubic yard for an itemized cost.
6. Order ready-mix by the cubic-yard figure (rounded up to the supplier's increment) or bagged mix by the rounded-up count.

Worked example

12 ft × 10 ft patio, 4 in thick, 10% waste:

• Thickness in feet: 4 ÷ 12 = 0.333 ft
• Cubic feet: 12 × 10 × 0.333 = 40 ft³
• Cubic yards: 40 ÷ 27 = 1.48 yd³
• Order volume (with 10% waste): 1.48 × 1.10 = 1.63 yd³
• 80 lb bags: ⌈40 ÷ 0.60 × 1.10⌉ = 74 bags
• 60 lb bags: 98 bags · 40 lb bags: 147 bags
• At $160 / yd³: 1.63 × $160 = $260.80

How to use this calculator

1. Tap a slab preset for typical thickness, or set thickness manually.
2. Enter length and width in feet, inches, yards, meters, or centimeters.
3. Adjust the waste factor and quantity.
4. Add a price per cubic yard if you want a cost estimate.
5. Use the bag counts to plan a trip to the home center, or the cubic-yard figure to schedule a ready-mix delivery.

Common mistakes

• Mixing units: thickness is usually in inches while length and width are in feet, the calculator converts but the wrong unit will silently 12× the result.
• Skipping waste: coming up short on a slab pour means a cold joint or a costly short-load fee.
• Rounding bags down: always round up to whole bags.
• Forgetting the sub-base: volume calculations assume the form is filled to the top with concrete. A poorly graded sub-base eats extra mix.

Frequently asked questions